1. Field of the Invention
The present invention relates generally to non-volatile magnetic memory and/or spintronic devices and particularly to magnetic random access memory (MRAM).
2. Description of the Prior Art
Computers conventionally use rotating magnetic media, such as hard disk drives (HDDs), for data storage. Though widely used and commonly accepted, such media suffer from a variety of deficiencies, such as access latency, higher power dissipation, large physical size and inability to withstand any physical shock. Thus, there is a need for a new type of storage device devoid of such drawbacks.
Other dominant storage devices are dynamic random access memory (DRAM) and static RAM (SRAM) which are volatile and very costly but have fast random read/write access time. Solid state storage, such as solid-state-nonvolatile-memory (SSNVM) devices having memory structures made of NOR/NAND-based Flash memory, providing fast access time, increased input/output (IOP) speed, decreased power dissipation and physical size and increased reliability but at a higher cost which tends to be generally multiple times higher than hard disk drives (HDDs).
Although NAND-based flash (or non-volatile) memory is more costly than HDD's, it has replaced magnetic hard drives in many applications such as digital cameras, MP3-players, cell phones, and hand held multimedia devices due, at least in part, to its characteristic of being able to retain data even when power is disconnected. However, as memory dimension requirements are dictating decreased sizes, scalability is becoming an issue because the designs of NAND-based Flash memory and DRAM memory are becoming difficult to scale with smaller dimensions. For example, NAND-based flash memory has issues related to capacitive coupling, few electrons/bit, poor error-rate performance and reduced reliability due to decreased read-write endurance. Read-write endurance refers to the number of reading, writing and erase cycles before the memory starts to degrade in performance due primarily to the high voltages required in the program, erase cycles.
It is believed that NAND flash, especially multi-bit designs thereof, would be extremely difficult to scale below 45 nanometers. Likewise, DRAM has issues related to scaling of the trench capacitors leading to very complex designs which are becoming increasingly difficult to manufacture, leading to higher cost.
Currently, applications commonly employ combinations of EEPROM/NOR, NAND, HDD, and DRAM as a part of the memory in a system design. Design of different memory technology in a product adds to design complexity, time to market and increased costs. For example, in hand-held multi-media applications incorporating various memory technologies, such as NAND Flash, DRAM and EEPROM/NOR flash memory, complexity of design is increased as are manufacturing costs and time to market. Another disadvantage is the increase in size of a device that incorporates all of these types of memories therein.
There has been an extensive effort in development of alternative technologies such as Ovanic Ram(or phase-change memory), Ferromagnetic Ram(FeRAM), Magnetic Random Access Memory (MRAM), Nanochip, and others to replace memories used in current designs such as DRAM, SRAM, EEPROM/NOR flash, NAND flash and HDD in one form or another. Although these various memory/storage technologies have created many challenges, there have been advances made in this field in recent years. MRAM seems to lead the way in terms of its progress in the past few years to replace all types of memories in the system as a universal memory solution.
High density magnetic random access memory (MRAM) has the potential to be the next generation storage device because of its unique advantages, such as non-volatility, i.e. preserving its stored values even when it is not receiving power, radiation hardness, high density, fast speed, and the like. MRAMs may be driven by magnetic field or by spin current. The latter has been known to attract a lot of attention due to its simplified design, reliability, and less cross talk. However, both of these MRAMs, i.e. spin current driven and magnetic field driven, will soon meet their writing power limitation with the memory cell size shrinking
Since a typical memory cell dimension of a memory cell made of MRAM is less than 100 nanometers (nm) for high density MRAM design, a high shape anisotropy or high magnetocrystalline anisotropy (Ku) material need be used in order to keep a relatively high Ku*V (or KuV) to resist thermal fluctuation, which acts to destroy the stored data. Therefore, the writing power (either through magnetic field or spin current, which highly depends on anisotropy energy constant Ku), has to be greatly increased to overcome the energy barrier between the two stable states. Such high writing power causes problems, particularly in MRAM memory elements having in-plane magnetic anisotropy, i.e. the magnetic moment of the free and fixed layers are parallel to the easy axis. Such problems include as poor compatibility with other electronic devices, high power consumption, and cross talk. However, unless the aspect ratio of a memory element made of the foregoing MRAM is large, thermal instability results. A high aspect ration is clearly undesirable because among other reasons, it prevents scalability and high density memory. Thermal instability is clearly undesirable because it causes unreliable memory.
Thus, there is a need for MRAM with spin current driven type switching (or spin torque transfer effect) with a relatively low switching current density and perpendicular magnetocrystalline anisotropy.